1. Field of the Invention
The invention relates generally to ophthalmic wavefront and topography measurement and more particularly to devices and methods for improved wavefront measurement using a sequential scanning technique, and to an apparatus and method for making real topography measurements.
2. Description of Related Art
Various ophthalmic diagnostic devices and techniques are known and available for mapping the physical and optical characteristics of the eye. Physical data such as corneal topology, pachymetry, refraction and other parametric data can be obtained from corneal topography systems such as the Orbscan II corneal topography system (Bausch & Lomb Incorporated, Rochester, N.Y.). Optical information such as the wavefront aberration of the eye can also be obtained from various devices and measurement methodologies. One such aberrometer uses a Hartmann-Shack wavefront sensor to measure ocular wavefront aberrations over the entire optical zone of the eye in a single pass. This is accomplished by illuminating a point on the retina with a very small diameter laser beam and focussing the outgoing light from the exit pupil of the eye with an array of lenslets onto a detector. Aberrations from the wavefront cause the focal spots on the detector that are created by the lenslet array to be displaced from the positions of an unaberrated wavefront passing through the lenslet array. These displacements allow the direct calculation of the wavefront error. Several well-known disadvantages of the Hartmann-Shack type device include dynamic range/resolution tradeoffs, low signal to noise ratios, suspect readings in pathologic eyes, and others known to those skilled in the art.
One of several alternative techniques for measuring wavefront aberrations derives from a psychophysical ray tracing approach originally attributed to Scheiner and based upon the Scheiner's disc concept. In summary, this concept is based on the adjustment of the direction of light from an image coming into the eye until the retinal image is aligned with the retinal image produced by a reference input light direction. A further explanation and more detailed description can be found in MacRae et aL, Customized Corneal Ablation, The Quest for Super Vision, Chapter 16, Slack Incorporated (2001). The Scheiner concept was further developed by Penney et al., and their device came to be known as the spatially resolved refractometer (SRR). The SRR operates by having a patient view a point object introduced to the eye at 37 selected positions on the cornea in a sequential fashion and asking the patient when tie image is focussed at a particular reference location as the directionality of the input object is changed. The resulting ray deviations provide wavefront slope information from which the wavefront can be determined.
A variant of the SRR concept adapted by Tracey Technologies LLC (Bellaire, Tex.) is referred to as sequential scanning or thin beam ray tracing. The sequential scanning technique relies on sequentially inputting a small diameter, collimated laser beam into the eye at selected points on the corneal surface and ultimately measuring the displacement (Δx,Δy) of each image spot on the retinal surface from a reference retinal spot location (x0, y0). The displacement errors are a direct measure of the transverse aberration for each particular point in the entrance pupil. With appropriate optics and relatively simple algebraic computing means, the displacements can be measured on a detector and the wavefront aberration calculated.
While the sequential scanning method for wavefront aberration measurement has certain advantages over alternate wavefront measuring techniques, this method suffers from some inherent shortcomings that principally relate to relying upon certain assumptions about the eye. These assumptions particularly relate to determining a correct length of the ocular bulbous; and, second, the assumption that the retinal surface is a flat plane at the posterior surface of the eye. In reality, however, the retinal surface is at best a curved envelope having a topography of irregular hills and valleys The inventors believe this to be especially evident in diseased retinas and at the foveal blind spot. Owing to the non-flat profile of the retina, the measurement of wavefront aberrations made using input beams that are parallel to a reference measurement axis such as the visual axis or the optical axis of the eye will lose accuracy as the retinal location of an image deviates from the retinal plane to follow the real retinal envelope profile.
Accordingly, the inventors have recognized a need for a way to improve the accuracy of the sequential scanning wavefront technique; and for a better understanding of the retinal topography around a retinal reference location and the ability to measure this topography.